No Build System

Occasionally some really small packages are shipped simply as a single
.c file. In these circumstances, you can either write your own
Makefile and ship it with the source tarball, or just manually
compile the thing from within the ebuild, preferebly explaining why.
Here's an example, from app-misc/hilite:

src_compile(){$(tc-getCC)${LDFLAGS}${CFLAGS} -o ${PN}${P}.c ||die}

Here's an example from x11-plugins/asclock, which ships with a
broken build system that doesn't actually work: